While Schottky barrier diodes (SBDs) have advantages such as very short reverse recovery time time (trr) and low forward voltage (VF), they have disadvantages such as high leakage current. Toshiba’s SiC SBDs overcome this drawback through use of an improved structure.
An SBD is formed by the junction of a semiconductor with a metal. It acts as a diode because of a difference in the work function between a semiconductor and a metal. Because the molecular structure may be discontinuous on the semiconductor–metal interface, irregularities on the surface, crystal defects, or other anomalies may occur. Current called leakage current (IR) flows when a high electric field is applied across a semiconductor-metal interface with these defects.
In SBDs with a conventional structure, the depletion region extends into the semiconductor side as shown below, causing the electric field produced by electric charge (or electrons) to be the strongest at the semiconductor-metal interface.
In contrast, in a JBS diode, the depletion region extends between p and n- regions that are partially buried below the semiconductor surface. When the reverse bias voltage increases, p-type depletion regions punch through each other and the position of the maximum electric field moves directly under the p region. This reduces the electric field on the surface where defects may be present thereby reducing leakage current.
When a conventional SBD is forward-biased, current flows through the following path: metal → Schottky barrier → Si (n-) → Si (n+). The Si (n-) layer has relatively large resistance because of low dopant concentration. Therefore, the IF - VF curve of this SBD looks like the one shown below.
Applications of SiC SBD include PFC circuits, which must be guaranteed to operate at high current because they are instantaneously exposed to large current during the turn-on of a power supply as well as during load variations. In that event, SBDs with an IF - VF curve like the one shown below might be overheated more than expected.
To address this problem, Toshiba has developed new SBDs with an improved JBS structure incorporating the concept of the Merged PiN Schottky (MPS) structure. The MPS structure has p+ regions buried in the n- region of an SBD as shown below. (In Toshiba’s design, part of the p-layer of the JBS structure (the shaded area in the figure) is enlarged and the impurity concentration of this part is increased.) The p+ regions and the n- region form a pn junction diode, which turns on when large current (surge current) is needed. This increases the current-carrying capability of the SBD, thereby reducing a rise in forward voltage even at high current and increasing the maximum allowable surge current value.
The MPS structure is characterized by the p+–n-–n+ configuration below the anode electrode.
At low current, the n- region typically has high resistance. However, when this SBD is forward-biased, holes and electrons flow into the n- region from the p and n regions respectively while maintaining electroneutrality. At this time, both holes and electrons exist in the n- region with high concentration. Consequently, the n- region acts like a heavily doped region, particularly at high current, exhibiting very low resistance (conductivity modulation). As a result, this SBD has an IF-VF curve as shown below, with low VF in the high-current region.
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